Technology of lithographic printing is based on immiscibility of oil and water. An oily material or a printing ink is preferentially received by an image area, and water or dampening solution is preferentially received by a non-image area. When a surface of a lithographic printing plate is wetted with water and then a printing ink is adhered, a background or a non-image area receives water and repels a printing ink, whereas, an image area receives a printing ink and repels water. Subsequently, the printing ink in the image area is transferred to an image-formed surface of materials such as papers, plastics and cloths through a roll.
In general, a lithographic printing plate precursor (the term “printing plate precursor” as used herein means a printing plate before imagewise exposure and development) and usually includes a photosensitive coating film applied on a substrate including aluminum as a base material. The coating film responds to light under exposure and the exposed area becomes soluble so that it is removed during development. Such printing plate precursor is called “positive-working”. Conversely, when exposed area of the coating film is cured by light so that it remains as the image area after development. Such printing plate precursor is called “negative-working”. Usually, the area, which is more soluble than the other areas of the coating film, is removed by using a strong alkali developer. The remaining image area receives the printing ink and is lipophilic, while the non-image area (background) receives water and is hydrophilic.
Usually, the lithographic printing plate precursor is developed by an automatic development processor after exposure. In the finishing step (or gum section) after development treatment, a plate surface protecting liquid (also referred to as a desensitizing liquid or a protective gum liquid) is applied on a printing surface of the printing plate with an image formed thereon. Application of the plate surface protecting liquid on the entire printing surface enables keeping of hydrophilicity of a support surface of the non-image area, and prevention of contamination of the printing surface due to adhesion of fingerprints, oils and fats, dusts, and the like during storage until use in a printing press after plate making, and during handling when mounted in the printing press, leading to further enhancement in scratch-resistant strength.
The lithographic printing plate thus finish-treated is mounted to a printing press and, after test printing called an operation of printing in the beginning, a normal printing operation is performed. The operation of printing in the beginning is usually performed until a printed matter with sufficient density can be obtained, and several tens to several hundreds of sheets are printed. The fewer the number of sheets used for test printing at the beginning of printing, more earlier a normal printing operation can be started, leading to saving of needless printing paper.
Japanese Translation of PCT International Application Publication No. 2008-511031 discloses a method for a post treatment of a lithographic printing plate, in which the lithographic printing plate is brought into contact with a solution containing at least one phosphono-substituted siloxane, so as to increase adhesion of an image area to a substrate and to prevent toning when a printing press is restarted.
Japanese Unexamined Patent Publication (Kokai) No. 2009-83106 discloses a plate surface protecting agent for lithographic printing plate using a compound having a specific ethylene oxide skeleton, which is excellent in the effect of preventing fingerprint smudge of the lithographic printing plate after plate making.
Japanese Unexamined Patent Publication (Kokai) No. 2004-243531 discloses a plate surface protecting agent comprising a polyalkylene oxide compound including a specific aromatic ring structure, which is excellent in the effect of preventing scratching and stains of a lithographic printing plate and the effect of protecting hydrophilicity of the non-image area, and also imparts satisfactory inking property to an ink in the printing process.
Japanese Unexamined Patent Publication (Kokai) No. 2000-89476 discloses a plate surface protecting agent comprising a polyalkylene oxide compound having a specific structure, which enables desensitization of the non-image area of a lithographic printing plate, and also causes no print omitting even when stored over the long period, leading to high stability.